Pesticides in soils as a source of residues in food after two years conversion to organic farming.

Pesticide residues in organic food can stem from non-intentional contamination or from fraudulent practices. The purpose of this study is to develop a pattern that allows to differentiate soil-derived pesticide residues in organic food from other sources. Using datasets from Germany for 90 pesticides, we tested four hypothesis concerning the statistical connection between sold quantities of pesticides, residues in soil and food, and chemical properties of the concerned substances. In a second step, based on rotational crop studies we identified, which pesticides could play a role as a source of residues in organic food, and tried to find a pattern for this purpose through principal component analysis (PCA) and linear discriminant analysis (LDA). Residues in soils are statistically connected to sold quantities and persistence in soil, but not to recommended application rates. Residues in food are significantly connected to residues in soil, but not to sold quantities nor to chemical properties associated with uptake of chemicals from soil. According to rotational crop studies, 21 out of 90 chemicals could possibly cause residues in organic food after the farm has undergone two years conversion, while for 69 chemicals this is unlikely. Using five variables, the PCA and LDA yield equations that allow to identify "possible" cases with an error rate of 1.5 % only, while the rate for "unlikely" is 71.4 %. We conclude that pesticide residues in soil are only a minor cause of residues in organic food. Our multi-step decision tree allows to identify cases where this could play a role.